The wildfire smoke that recently suffocated cities from Athens to New York may be the most visible sign of air pollution caused by heatwaves.
But extreme heat can also induce a host of other chemical processes that are hazardous for human health, the World Meteorological Organization (WMO) said in its annual Air Quality and Climate Bulletin.
"Heatwaves worsen air quality, with knock-on effects on human health, ecosystems, agriculture and indeed our daily lives," WMO chief Petteri Taalas said in a statement.
A new study by the Energy Policy Institute at the University of Chicago suggested that fine particulate pollution from sources such as vehicle and industrial emissions, sand and wildfires is "the greatest external threat to public health" worldwide.
"Climate change and air quality cannot be treated separately," Taalas stressed.
"They go hand-in-hand and must be tackled together to break this vicious cycle."
- 'More extreme' -
While Wednesday's report was based on 2022 data, Taalas cautioned that in terms of temperatures, "what we are witnessing in 2023 is even more extreme".
On Wednesday, the European Union's Copernicus climate monitor said 2023 was likely to be the hottest year in human history, after the last three months were the warmest ever recorded.
That, in turn, is potentially bad news for air quality.
"Air quality and climate are interconnected because the chemical species that affect both are linked," the WMO said.
"The substances responsible for climate change and for the degradation of air quality are often emitted by the same sources, and... changes in one inevitably cause changes in the other."
It pointed for instance to how the combustion of fossil fuels emits carbon dioxide and nitrogen oxide into the atmosphere.
These are not only heat-trapping greenhouse gases but potential precursors of pollutants such as ozone and nitrate aerosols.
- Inter-linked -
Researchers meanwhile widely agree that climate change is causing more intense and more frequent heatwaves, and that this in turn is leading to a growing risk of more severe wildfires, WMO said.
"Heatwaves and wildfires are closely linked," said Lorenzo Labrador, a WMO researcher at the Global Atmosphere Watch network which compiled Wednesday's Bulletin.
"Smoke from wildfires contains a witch's brew of chemicals that affects not only air quality and health, but also damages plants, ecosystems and crops -- and leads to more carbon emissions and so more greenhouse gases in the atmosphere," he said in the statement.
He stressed though that "it is yet too early to say" if 2023 would prove worse in terms of atmospheric pollution than last year.
"Even though this has been a record-breaking wildfire season, in particular in Europe and western Canada, ... the relationships and interactions and chemical processes that link climate change to atmospheric pollution are not linear," he told reporters in Geneva.
The 2022 data detailed in the report showed how heatwaves last year triggered wildfires in the Northwestern United States, leading to unhealthy air.
Soaring temperatures in Europe, accompanied by unusually high amounts of desert dust reaching the continent, meanwhile led to increased concentrations of both particulate matter and ground-level ozone, it said.
Stratospheric ozone helps to shield humans and vegetation from harmful ultraviolet rays from the sun.
But at ground level, where it is generated by a reaction between traffic fumes and sunlight, the gas attacks lung tissue, causing chest pain, coughing and shortness of breath.
It also reduces crop yield, with ozone-induced losses averaging 4.4-12.4 percent globally for staple food crops, and wheat and soybean losses as high as 15-30 percent in parts of India and China.
2023 likely to be hottest year on record
Paris (AFP) Sept 6, 2023 -
2023 is likely to be the hottest year in human history, and global temperatures during the Northern Hemisphere summer were the warmest on record, the EU climate monitor said on Wednesday.
Heatwaves, droughts and wildfires struck Asia, Africa, Europe and North America over the last three months, with dramatic impact on economies, ecosystems and human health.
The average global temperature in June, July and August was 16.77 degrees Celsius (62.19 degrees Fahrenheit), surpassing the previous 2019 record of 16.48C by a wide margin, the European Union's Copernicus Climate Change Service (C3S) said in a report.
"The three months that we've just had are the warmest in approximately 120,000 years, so effectively human history," C3S deputy director Samantha Burgess told AFP.
Last month was the hottest August on record and warmer than all other months except July 2023.
"Climate breakdown has begun," said UN Secretary General Antonio Guterres, echoing famous testimony before the US Congress 35 years ago, in which government scientist James Hansen declared that global warming had begun.
"Our climate is imploding faster than we can cope," Guterres added.
Also on Wednesday, the World Meteorological Organization warned that more frequent and intense heatwaves are generating a "witch's brew" of air pollution that shortens human lifespans and damages other life forms.
"Heatwaves worsen air quality, with knock-on effects on human health, ecosystems, agriculture and indeed our daily lives," WMO chief Petteri Taalas said in a statement.
Record-high global sea surface temperatures played a major role in stoking heat throughout the summer, with marine heatwaves hitting the North Atlantic and the Mediterranean Sea.
"Looking at the additional heat we have in the surface ocean, the probability is that 2023 will end up being the warmest year on record," Burgess said.
If the Northern Hemisphere has a "normal" winter, "we can almost virtually say that 2023 will be the warmest year that humanity has experienced," she added.
- Warming oceans -
Oceans have absorbed 90 percent of the excess heat produced by human activity since the dawn of the industrial age, according to scientists.
This excess heat continues to accumulate as greenhouse gases -- mainly from burning oil, gas and coal -- build up in the Earth's atmosphere.
Excluding the polar regions, global average sea surface temperatures exceeded the previous March 2016 record every day this summer from July 31 to August 31.
Warmer oceans are also less capable of absorbing carbon dioxide (CO2), exacerbating the vicious cycle of global warming as well as disrupting fragile ecosystems.
Antarctic sea ice remained at a record low for the time of year with a monthly value 12 percent below average, "by far the largest negative anomaly for August since satellite observations began" in the 1970s, C3S said.
Higher temperatures are likely on the horizon: the El Nino weather phenomenon -- which warms waters in the southern Pacific and beyond -- has only just begun.
Scientists expect the worst effects of the current El Nino to be felt at the end of 2023 and into next year.
- 'Wake up call' -
Scientists reacted strongly to the C3S report.
"2023 is the year that climate records were not just broken but smashed," said Mark Maslin, a professor of climatology at University College London.
"Extreme weather events are now common and getting worse every year -- this is a wake up call to international leaders."
"Global warming continues because we have not stopped burning fossil fuels -- it is that simple," said Friederike Otto, a climate scientist at Imperial College London.
At the 2015 Paris climate summit, countries agreed to keep global temperature increases to "well below" 2C above pre-industrial levels, with an aspirational target of 1.5C.
A "Global Stocktake" by UN experts due this week assessing the world's progress in meeting these goals will confirm that current national carbon-cutting commitments fall far, and would see Earth's surface warm 2.7C.
The C3S findings came from computer-generated analyses using billions of measurements from satellites, ships, aircraft and weather stations around the world.
Proxy data such as tree rings and ice cores allow scientists to compare modern temperatures with figures before records began in the mid-19th century.
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